Energy storage and renewables
Hold on - is burning biomass
bad for the climate?
Is replacing coal burning with biomass good for climate change or, as some have suggested,
does it actually create more emissions? John D Sterman argues the latter, based on a
recently published lifecycle analysis of US forests.
greenhouse gas causing global
century at least - even when wood
displaces coal, the most carbon-
intensive fuel.
• Burning wood adds carbon
dioxide to the atmosphere.
Repaying carbon debts
• Carbon dioxide is only removed
Although wood has approximately
from the atmosphere if the
the same carbon intensity as
forests harvested to supply
coal per kwh, the combustion
wood bioenergy grow back and
and processing efficiencies of
keep that carbon sequestered in
wood pellets, the dominant wood
biomass and soils.
bioenergy in the EU and the UK,
are lower. Consequently, pellet-
• Regrowth takes time.
fired power plants generate more
• Regrowth is not certain. Fire,
carbon dioxide per kWh than coal.
insect damage, re-harvest, or
Burning wood instead of coal
ood bioenergy is booming
conversion to other uses (eg
therefore creates a carbon debt - an
as governments around
agriculture, development) can
immediate increase in atmospheric
the world struggle to cut
limit or prevent forest recovery.
carbon dioxide compared to fossil
their greenhouse gas emissions.
The European Union declared all
The climate impact of wood and
That carbon debt can be repaid
biofuels to be carbon neutral to help
other biofuels therefore depends on
over time - if the forest regrows. To
meet its goal of 20% renewable
some critical questions.
illustrate, Figure 1 depicts a
energy by 2020, triggering a surge
First, at the point of combustion,
simulation showing what happens
in wood use. The UK subsidises
do biofuels generate more or less
to the carbon stored in a hardwood
wood pellets for electric power
carbon dioxide per unit of end-use
forest in the southern US after
generation and is now the world’s
energy than the fuel they displace?
clear-cut (where all trees are cut
largest pellet importer. The US has
Emissions accounting must consider
just declared forest bioenergy to be
emissions both from combustion
Clearcutting removes nearly all
carbon neutral, while at the 2017
and from the fuel supply chain.
the biomass, burning it for
UN climate summit 19 nations
Second, how much carbon can
bioenergy adds carbon dioxide to
including Brazil, Canada, China,
the harvested land remove from the
the atmosphere, accelerating
India and Indonesia joined the
atmosphere as it grows back, how
climate change. Lacking sufficient
Biofuture Platform initiative to
long does regrowth take, and how
biomass to move carbon from leaves
increase use of ‘sustainable
do the answers to these questions
to roots, the stock of carbon stored
depend on the fate of the harvested
in the soil falls for several decades
But are biofuels really carbon
even as trees start to grow back.
neutral? Is wood really good?
To answer these questions my
Biomass and soil carbon gradually
The appeal is intuitive: burning
colleagues and I extended the
recover, yet after 100 years the total
fossil fuels injects carbon
widely-used C-ROADS climate policy
carbon stored in the forest will still
sequestered in geological reservoirs
model to explore the dynamic
remain below the initial level.
for millions of years into the
impact of biofuels on carbon
This example is typical. We
atmosphere, causing global
emissions and climate. The model is
examined a wide range of forests in
warming. In contrast, biofuels
fully documented and freely
the US, the largest supplier of pellets
recycle carbon from the atmosphere,
available. Our research is
to the UK. Carbon debt payback
so, people argue, there are no net
independent, funded neither by
times averaged 87 years after
bioenergy producers, who support
clear-cut, and 63 years when forests
Unfortunately, science shows
wood use, nor by environmental
are thinned.
otherwise. Burning wood to produce
groups, some of which oppose it.
These payback times are
energy can actually worsen climate
Indeed, given the urgency and
optimistic: we assume all land
change, at least through the year
magnitude of emissions reductions
harvested for bioenergy recovers
2100 - even if wood displaces coal,
needed to limit global warming to
without re-harvest, unplanned
the most carbon-intensive fuel.
no more than 2°C, my sincere hope
logging or conversion to other uses,
Why, and how do we know?
was that wood would prove to be
and without damage from fire,
part of the climate solution.
insects, severe weather or other
Supply and regrowth
Unfortunately, our results
ecological disturbances that would
An example of clearcutting
First, some basic physics:
in North Carolina in 2017
indicate that wood worsens climate
harm carbon uptake or inject
• Carbon dioxide is the principal
change through the rest of this
greenhouse gases into the
Photo: Dogwood Alliance
Energy World | May 2018 29
Energy storage and renewables
fuel. Carbon dioxide emissions per
unit of primary energy from wood
are about the same as coal, but 30%
higher than fuel oil and 80% higher
than natural gas.
We assume that the decline in
coal use resulting from wood does
not lower coal prices, and increases
coal demand elsewhere. Such
rebound coal demand would mean
the carbon debt from burning wood
would never be repaid, permanently
worsening global warming. Finally,
we don’t consider ecological
damage from harvesting wood,
including erosion and soil loss,
habitat fragmentation and resulting
wildlife loss, or declines in hunting,
recreation and tourism.
Of course, these results do not
support continued coal use. To have
any decent chance of limiting global
warming to no more than 2°C,
global greenhouse gas emissions
must start to decline in the next few
Figure 1. Simulation showing the recovery of an oak-hickory forest in the southern US after clear-cut - assuming the
land is not converted to pasture, agriculture, or other uses, not re-harvested, and suffers no fire, insect damage or other
years. Solar and wind with storage,
events that could harm regrowth
and especially energy efficiency, are
Source: John Sterman, Lori Siegel, Juliette Rooney Varga,
the cheapest, safest, and quickest
ways to cut emissions while
providing the goods and services
atmosphere. It is worth noting that
released when they are burned.
people need. These technologies
climate change worsens fire, insect,
Could this be mitigated if forests
have a far smaller footprint than
and severe weather risks.
are managed sustainably, by
forest bioenergy and immediately
thinning instead of clear-cut? Large
lower emissions, without the need
Taking out more credit
pellet users and suppliers claim that
to wait for uncertain regrowth of
The International Energy Agency
they never cause deforestation and
forest lands.
and the bioenergy industry project
source low-grade wood including
Government policies should not
substantial growth in wood use for
tree tops, branches, and thinnings
violate basic laws of physics.
decades to come. Such expansion
(small trees). These claims are
Declaring that wood biofuels are
would steadily add to atmospheric
carbon neutral, as the EU, UK and
carbon dioxide, even if the
But analysis indicates that wood
others have done, assumes regrowth
harvested lands fully recover.
bioenergy worsens climate change
is rapid and certain. Neither is true.
To see why, consider the national
even if never sourced by
This accounting fiction promotes
debt of the United States. The US
clearcutting. We examined scenarios
costly policies that accelerate
repays the bonds it issues in full at
in which all wood bioenergy is
climate change. Through renewable
maturity. However, because the
harvested by thinning. Although
energy subsidies, the UK and Europe
government runs a deficit, new
less damaging than clear-cut, the
are paying power plants to make
borrowing exceeds repayment of
average carbon debt payback time
climate change worse.
maturing debt, increasing the
for the US forests we examined was
Proper accounting would tally
national debt. Similarly, every year
still 63 years, and the resulting
the emissions from all sources of
wood biofuel use grows, the carbon
climate damage persists for
energy, whether coal, gas, solar or
dioxide added to the atmosphere
centuries even if the carbon debt
wood. For wood and other biofuels,
when that wood is burned exceeds
from thinning is fully repaid.
offsetting reductions in atmospheric
the carbon removed by regrowth,
What if wood pellets are sourced
carbon dioxide should be credited
increasing atmospheric carbon
from fast-growing managed tree
only when and if there is net new
dioxide even if all carbon debt is
farms instead of slow-growing
growth on the lands harvested to
eventually repaid in full.
natural forests? Counter to
supply the biomass.
Critically, during the decades
intuition, harvesting existing forests
Our results
Instead of subsidies that harm
before the carbon debt is repaid, the
and replanting with fast-growing
indicate that
the climate, a market-based
additional carbon dioxide in the
species in managed plantations
approach combining accurate
wood worsens
atmosphere from wood bioenergy
worsens the climate impact of wood
accounting with a meaningful price
worsens climate change. Higher
biofuel. The carbon density of
climate change
on carbon and other greenhouse gas
carbon dioxide concentrations
managed plantations is lower than
through the rest
emissions - no matter where and
resulting from wood use then
unmanaged forests, so the carbon
of this century at
how they arise - would lead to far
accelerates global warming, raising
sequestered in plantations never
larger emissions cuts at far lower
least - even
sea level faster and intensifying
offsets the carbon taken from the
cost. l
other damages including ocean
original forest. Converting forests to
when wood
acidification, the incidence of
plantations permanently worsens
displaces coal,
John D Sterman is the Jay W Forrester
extreme weather, water stress and
climate change.
Professor of Management at the MIT
the most carbon-
crop yield decline.
Sloan School of Management, Cambridge,
intensive fuel
These impacts persist for
Accounting fiction
Massachusetts, US, and Director of the MIT
System Dynamics Group. The paper on the
centuries or more. They are not
The assumptions of our study
above research, published in Environmental
reversed even if the forests
favour bioenergy. Specifically, we
Research Letters, is available at
harvested for bioenergy eventually
assume wood is used to offset coal,
take up all the carbon dioxide
the most carbon-intensive fossil
Energy World | May 2018 31